According to the related art, technologies for detecting illuminance using a solid-state imaging element such as a complementary metal oxide semiconductor (CMOS) image sensor have been suggested. For example, camera modules can adjust the lightness of a display screen in accordance with the detected illuminance. As a method of detecting illuminance by a solid-state imaging element, for example, there is a known a method of aggregating luminance values detected with pixels and setting the average of the luminance values on the entire imaging screen as an illuminance value.
In a CMOS image sensor, a dynamic range obtained under an optimum exposure condition is generally a range from 10 bits to 12 bits due to the restrictions on conversion accuracy in an analog-to-digital converter (ADC) for each pixel, the number of saturated electrons of a pixel, or the like. On the other hand, a general illuminance sensor can acquire illuminance values of a dynamic range of about 16 bits. Accordingly, in CMOS image sensor according to the related art, a technology for converging an electronic shutter (ES) time or an analog gain (AG) on an optimum condition under which an illuminance value is calculated has been disclosed to acquire the illuminance values of the same dynamic range as that of the illuminance sensor.
For example, when a gray scale disappears due to so-called overexposure in which saturation of an output occurs with respect to the amount of incident light from a subject, an illuminance value is measured by suppressing the overexposure by shortening the ES time or reducing the AG. When a gray scale disappears due to so-called underexposure in which an output remains to be in a black level with respect to a change in the amount of incident light from a subject, an illuminance value is measured by suppressing the underexposure by increasing the ES time or the AG. The CMOS image sensor converges into an optimum condition by gradually updating the ES time and the AG.
According to the technology of the related art, for example, in regard to a change in an illuminance condition such as movement of a camera, the ES time and the AG are updated about several tens of times to converge the condition. In order to acquire an accurate illuminance value, a wait is necessary until the convergence of the condition to suppress the overexposure and underexposure as much as possible. Therefore, it takes long time to acquire the illuminance value. As a frame rate is further decreased to reduce power consumption, it takes longer time until the acquisition of the illuminance value. Further, the time until the acquisition of the illuminance value is frequently changed, since the number of time the ES time and the AG are updated is varied in accordance with an imaging condition or the like.